Lasers, while a relatively new phenomenon, have become relatively commonplace in technology. Continuous and pulse lasers are well known in laser applications. See, for example, U.S. Pat. No. 3,721,756, issued Mar. 20, 1973, to C. E. Baker with regard to continuous lasers. For pulse lasers, see, for example, U.S. Pat. Nos. 4,206,347, issued June 3, 1980 to Avicola; and 4,308,507, issued Dec. 29, 1981, to Pelasance, et al. A relatively new development in pulse lasers are metal vapor lasers. See, for example, U.S. Pat. Nos. 3,831,107, issued Aug. 20, 1974, to Karras; 4,048,587, issued Sept. 13, 1977 to Liu, et al; 4,247,630, issued Jan. 27, 1981, to Karras, et al; 4,295,103, issued Oct. 13, 1981, to Ljudmirsky; and 4,328,464, issued May 4, 1982 to Pivirotto. For further discussions about pulsed lasers, see also U.S. Pat. Nos. 4,107,701, issued Aug. 16, 1978, to Sprague, et al; 3,725,812, issued Apr. 3, 1973, to Scott; 3,896,397 issued July 27, 1975, to De Wit, et al; 3,904,987, issued Sept. 9, 1975, to Cheng; 3,936,769, issued Feb. 3, 1976, to De Wit, et al; U.S. Pat. No. Re 29421, issued Sept. 27, 1977, to De Wit, et al; U.S. Pat. Nos. 3,982,203, issued Sept. 21, 1976; 4,057,770, issued Nov. 8, 1977, to Henningsen, et al; 4,105,953, issued Aug. 8, 1978, to Jernigan; 4,227,159, issued Aug. 8, 1978, to Jernigan; 3,392,260, issued July 9, 1968, to Dernback; 3,465,358, issued Sept. 2, 1969, to Bridges; 3,506,928, issued Apr. 14, 1970, to Korpel; 3,544,916, issued Dec. 1, 1970, to Angelbeck; 3,566,303, issued Feb. 23, 1971, to De Maria; 3,613,024, issued Oct. 12, 1971, to Geusic; and 3,916,338, issued Oct. 28, 1975, to Jensen, et al.
The use of lasers in the production of images, and in particular, the production of a sequential set of electrical signals which represents an original picture for direct display through the use of lasers ("video imaging") is also know in the art. See, for example, U.S. Patents to Baker, supra; U.S. Pat. Nos. 3,737,573, issued June 5, 1973 to Kessler; 3,818,129, issued June 18, 1974 to Yamamoto; 3,958,863, issued May 25, 1976, to Isaacs, et al; 3,977,770, issued Aug. 31, 1976 to Isaacs, et al; 3,994,569, issued Nov. 30, 1976, to Isaacs, et al; U.S. Pat. No. 3,636,251, issued Jan. 18, 1976, to Daly, et al. See also, "High-Quality Laser Color Television Display", by Taneda, et al, reprinted from the Journal of the Society of Motion Pictures and Television Engineers, June 1973, Volume 82, No. 6; "A 1125 Scanning-Line Laser Color TV Display" by Taneda, et al, published and presented at the 1973 SID International Symposium and Expedition; and "Laser Displays" by Yamamoto, reprinted from Advances and Image Pick-up and Display, Volume 2 of the Academic Press, Inc. in 1975. For general references to video imaging, see U.S. Pat. Nos. 3,507,984, issued Apr. 21, 1970, to Stavis; 3,727,001, issued Apr. 10, 1973, to Gottlieb; and 3,636,251, issued Jan. 18, 1972, to Daly, et al.
It is also known in the art to use isotropic Bragg cells and acoustic-optical modulation with video imaging systems as discussed in the patent to Yamamoto, supra, the patents to Isaacs, supra, and the articles supra.
Because of the short pulse duration and high average power, Nd:Yag Q-switched lasers were chosen by Yamamoto. The infra red light emission, which was converted into second harmonic waves by using an appropriate non-linear optical crystal, provided the necessary visible light. For example, the 1.06-micron spectral line emission is converted into green light which has a wave length of approximately 0.534 microns. The 1.318-micron spectral line emission is converted into red light which has a wave length of approximately 0.660 microns. Because of the impractical operation, the 0.946-micron spectral line emission was not used to obtain the blue line. Instead, optical mixing of 1.32 microns and 6.60 microns was converted into an additional wave length of approximately 4.39 microns.
Although the theory of second harmonic generation and parametric mixing in non-linear crystals appears to be a realistic solution, the practical limitations are too severe. The prior art examples show a situation that uses a luminous flux of one thousand lumens, with two watts of green and approximately ten watts each of red and blue. The conversion efficiency of the non-linear crystals is poor, being less than twenty percent in the best of cases. Ten watts of red couldn't be obtained; ten watts of blue could hardly be achieved. The amount of radiation including on the crystal would have to be of significant energy level to achieve the required light; achieving this would lead to the destruction of the non-linear crystals. The luminous efficiency of the red and blue lines, when taking the spectral response curve of the human retina into consideration, is so low that the amount of light needed is in excess of the limits imposed by the Nd:Yag laser family, although the reproducable color spectrum is excellent for low luminosity.
Because of the low luminous efficiency of red 0.660 microns and blue 0.440 microns, dye laser system were proposed in the prior art. The green 0.532-micron emission was used as the pumping source and a solution of a florescent dye was used as the active medium, to produce the red laser light which is tunable, allowing the selection of a spectral line having relatively high luminosity. This does not really solve the problem because twice as much green is now needed, and the relatively poor efficiency incurred by the addition of pumping a dye cell is a disadvantage. Similarly, blue light which is tunable requires ultra-violet light, which has a wave length of approximately 0.350 microns, to pump a dye laser. The ultra-violet was achieved by parametric mixing of 1.06 microns and the second harmonic 0.534 microns, the sum frequency being 0.350 microns. This requires, however, additional green light in order to produce the blue. Accordingly, prior art systems make an impractical projector. Although the Nd:Yag has relatively good overall energy to light efficiency, the applicable mixing hinders the practical application.
For general references to Bragg cells, both anisotropic and isotropic, including shear cut and shear wave propagation of radio frequencies, sometimes referred to as shear Bragg defraction, see U.S. Pat. Nos. 3,644,015, issued Feb. 22, 1972, to Hearn; 3,653,765, issued Apr. 4, 1972, to Hearn; 3,701,583, issued Oct. 21, 1972, to Hammond; 4,052,121, issued Oct. 4, 1977, to Chang; 4,110,016, issued Aug. 29, 1978, to Derg, et al; 4,126,834, issued Nov. 21, 1978, to Coppock; 4,201,455, issued May 6, 1980, to Vadasz, et al; 4,339,821, issued July 13, 1982, to Coppock, et al; 4,342,502, issued August 3, 1982, to Chang; 3,828,276, issued Aug. 6, 1974, to Cohen; 4,083,976, issued June 6, 1978, to Das; 3,485,559, issued Dec. 23, 1969 to De Maria; 4,371,964, issued Feb. 1, 1983, to Podmaniczky, et al; 3,389,348, issued June 18, 1968, to De Maria; 3,749,476, issued July 31, 1973, to Daly, et al; 4,000,493, issued Dec. 28, 1976, to Spaulding, et al; and 4,016,563, issued Apr. 5, 1977, to Pedinoff. For Bragg cells, perhaps used in video imaging systems, see U.S. Pat. No. 3,935,566, issued Jan. 27, 1976, to Seopko.
Generally, see also U.S. Pat. Nos. 4,115,747, issued Sept. 19, 1978, to Sato, et al; 4,229,079, issued Oct. 21, 1980, to Wayne, et al; 4,308,506, issued Dec. 29, 1981, to Ellis; 4,337,442, issued June 29, 1982, to Mauck; 3,633,995, issued Jan. 11, 1972, to Lean; and 3,711,791, issued Jan. 16, 1973, to Erickson; and 4,130,834, issued Dec. 19, 1978, to Mender, et al.
However, none of the prior art set out above discloses a system of a video projector which can equal the present system in such parameters as maximum light output capacity and energy-to-light conversion efficiency.